Various processes for forming ceramic molded articles by hot-isostatic compression are known.
In one known method, a powder preform is injection molded or extruded cold-isostatically, thereafter provided with a gas-impervious coating, and then compressed hot-isostatically.
In another known process of manufacture, the prefabricated powder preform is sintered, the density remaining below about 95%, so that the sintered preform remains porous. The body is then provided with a gas-impervious coating and hot isostatically compressed.
In a third known method of manufacture a powder preform is sintered to a density of more than 95% so that it is no longer porous and it is then hot-isostatically compressed.
Glass is ordinarily used for the coating in the first two methods of manufacture.
In the prior art, it is difficult to remove the coating. Furthermore, there is the disadvantage that the temperature at which the hot-isostatic compression is carried out is limited to 1700.degree. C. to 1850.degree. C. due to the fact that the glass softens at higher temperatures. The powder preform contains sintering promoting additives which impair the high temperature strength of the body (for instance, MgO or Y.sub.2 O.sub.3 in Si.sub.3 N.sub.4) or the strength (for instance, B or Al or C in SiC) or the heat conductivity (for instance, B or Al or C in SiC). Sintering promoting additives are, in principle, necessary in order to permit the hot-isostatic compression at a temperature which is still withstood by the glass coating (in accordance with the first two known methods of manufacture) or in order to obtain the desired sintering (in accordance with the last two methods of manufacture). A minimal amount of sintering promoting additives is desirable in order to avoid the aforesaid disadvantages.